Chemical Equations

Chemical Equations

• Reactants – the substances that exist before a chemical change (or reaction) takes place.

• Products – the new substance(s) that are formed during the chemical changes.

• CHEMICAL EQUATION indicates the reactants and products of a reaction.

REACTANTS PRODUCTS

Showing Phases in Chemical Equations

Solid Phase – the substance is relatively rigid and has a definite volume and shape. NaCl(s)

Liquid Phase – the substance has a definite volume, but is able to change shape by flowing. H2O(l)

Gaseous Phase – the substance has no definite volume or shape, and it shows little response to gravity. Cl2(g)

H2O(s) H2O(l) H2O(g)

Additional Symbols Used in Chemical Equations

“Yields”; indicates result of reaction

Used to indicate a reversible reaction

A reactant or product in the solid state; also used to indicate a precipitate

Alternative to (s), but used only to indicate a precipitate

A reactant or product in the liquid state

A reactant or product in an aqueous solution (dissolved in water)

A reactant or product in the gaseous state

(s)

(l)

(aq)

(g)

Additional Symbols Used in Chemical Equations

Alternative to (g), but used only to indicate a gaseous product

Reactants are heated

Pressure at which reaction is carried out, in this case 2 atm

Pressure at which reaction is carried out exceeds normalatmospheric pressure

Temperature at which reaction is carried out, in this case 0 oC

Formula of catalyst, in this case manganese (IV) oxide, used to alter the rate of the reaction

2 atm

pressure

0 oC

MnO2

Describing a Chemical Reaction

Indications of a Chemical Reaction

– Evolution of heat, light, and/or sound

– Production of a gas

– Formation of a precipitate

– Color change

Chemical Equations

Depict the kind of reactantsreactants and productsproducts and their relative amounts in a reaction.

4 Al(s) + 3 O2(g) 2 Al2O3(s)

The numbers in the front are called stoichiometric coefficientsstoichiometric coefficients.

The letters (s), (g), and (l) are the physical states of compounds.

reactantsreactants productproduct

aluminum oxide

Chemical EquationsBecause the same atoms are present in a reaction at the beginning (reactants)and at the end (products), the amount of matter in a system does not change.

The Law of Conservation of MassLaw of Conservation of Mass

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Chemical Chemical FactoryFactory

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Chemical Equations

Because of the principle of the conservation conservation of massof mass, all equations must be balanced.

There must be the same number of atoms of There must be the same number of atoms of the same kind on both sides.the same kind on both sides.

Lavoisier, 1788

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Visualizing a Chemical Reaction

Na + Cl2 NaCl

___ mole Cl2 ___ mole NaCl___ mole Na

2

10 5 10

2

10 5 10

Balancing Chemical Equations

Balanced Equation – one in which the number of atoms of each element as a reactant is equal to the number of atoms of that element as a product

What is the relationship between conservation of mass andthe fact that a balanced equation will always have the same number of atoms of each element on both sides of an equation?

Determine whether the following equation is balanced.

2 Na + H2O 2 NaOH + H2

2 Na + 2 H2O 2 NaOH + H2

Balancing Chemical Equations

• Write a word equation for the reaction.

• Write the correct formulas for all reactants and products.

• Determine the coefficients that make the equation balance.

Balancing Chemical Equations

Other examplesNO(g) + O2(g) NO2(g) is it balanced?

Is this balanced? NO(g) + O(g) NO2(g)

Is this OK?

NO(g) + ½ O2(g) NO2(g)Is this balanced?

Is this OK?

Balancing Chemical Equations

An important point to remember2 NO(g) + O2(g) 2NO2(g)

The 2 to the left of NO(g) and NO2(g) refers to the number of molecules present in the balanced equation.

It is a “multiplier” for every atom in the molecule.

The subscript 2 in O2 (g) and NO2(g) refers to the number of atoms of this type that are present in each molecules (or ionic compound).

(NH4)3PO4 + Mg(OH)2 Mg3(PO4)2 NH4OH?

ammonium phosphate magnesium hydroxide magnesium phosphate ammonium hydroxide

NH41+ OH1-

+ 62 3

Now you try…

AlCl3 + Li2CO3 Al2(CO3)3 + LiCl32 6

1) Write a word equation for the reaction.

Write a balanced equation for the reaction between chlorineand sodium bromide to produce bromine and sodium chloride.

2) Write the correct formulas for all reactants and products.

3) Determine the coefficients that make the equation balance.

chlorine + sodium bromide bromine + sodium chloride

Cl2 + NaBr Br2 + NaCl

Cl2 + 2 NaBr Br2 + 2 NaCl

1) Write a word equation for the reaction.

2) Write the correct formulas for all reactants and products.

3) Determine the coefficients that make the equation balance.

aluminum sulfate + calcium chloride calcium sulfate

Al2(SO4)3 + CaCl2 CaSO4 + AlCl3

Write the balanced equation for the reaction between aluminum sulfate and calcium chloride to form a white precipitate of calcium sulfate.

Al2(SO4)3 + 3 CaCl2 3 CaSO4 + 2 AlCl3

+ aluminum chloride? ?

CH4 + 2 O2 CO2 + 2 H2O

Reactants Products 1 C atom 1 C atom 4 H atoms 4 H atoms 4 O atoms 4 O atoms